Buy an arch. You will use this as a base for attaching balloons. Ready-made arches can be found in various sizes at garden supply and rental shops. An arch made of thin wire is a safe bet. Just make sure the arch is wide and tall enough to meet your needs a backyard birthday party and a wedding reception are likely to require arches of very different size.

Alternatively, make an arch. If you want or need to make your own arch instead, use rugged, flexible plastic tubing and two cinder blocks. (Other anchor-weight items, such as heavy-duty plastic buckets or table umbrella bases, can also be used.) Place the cinder blocks at either edge of the arch, and bend a length of tubing, such as thin-walled PVC pipe, into each block so that an arch is formed between them. Fill in the holes in each cinder block with sand or gravel to add stability.

Even with a sand-filled base, a tube arch may be prone to flopping over. If this seems like it will be a problem, look for nearby trees or posts to which you can tie each side of the tubing with festive ribbon. Pull the ribbons taut before tying them to ensure maximum stability.

To ensure an arch of proper size, buy more tubing than you think you will need and begin with all of it. Each time you check the size of the arch, if it is too large, take one end of the tube out of a cinder block and saw off about 6 inches, then readjust the arch and check it again until you reach the size you want.

Inflate your balloons. For this type of arch, either helium or normal air can be used, as the arch will stay upright independently of the balloons. Inflate half a dozen or so balloons to start and eyeball how they will fit around the arch, then make a rough estimate of how many you will need and finish inflating. Remember that the balloons should go all around the arch and hide most of the structure from view.

Fasten your balloons. Using string or adhesive tape, secure the base of each balloon to a free spot on the arch, starting at one end and working towards the other to avoid missing any spots. Wrap colorful ribbon around the arch to hide the tape or string. Reserve leftover balloons to be used as decorative elements elsewhere, or to replace any balloons on the arch that pop. Your arch will be bright and colorful, rustling in the breeze while remaining stationary overall.

Acquire rope. Purchase the lightest weight of rope you trust to keep a garland of helium balloons secure, because this is exactly what the rope will be used for. Lay out the rope on the ground in an arch shape and adjust it until you have the size you want for the finished arch, then cut it to the desired length plus a few extra feet so that it can be tied securely at both ends.

To save yourself trouble later, consider tying a loop into each end of your rope right now for easy fastening.

Twine or even fishing line can be used for smaller displays. Parachute cord or thin nylon rope is better for larger arches.

Tie it down. Secure one end of the rope to a base element. As with the plastic tubing method outlined above, a cinder block is a cheap, reliable, and portable anchor. Landscape and accent elements of appropriate weight or tenacity, such as trees or statuary, can also be used if they are available and it makes sense to use them. Be sure the rope is fastened firmly to prevent it from floating away. The other end of the rope can remain unsecured for now.

Inflate and attach your balloons. Using a helium tank, fill one balloon at a time and then take it to the rope and firmly attach it. Florist's wire is a good choice for securing helium balloons, as it will not stretch or loosen. Strong adhesive tape such as electrical or duct tape can also be used.

Have plenty of attaching material on hand and use plenty of length to tie down each balloon, wrapping the wire or tape several times to prevent loosening.

As before, work methodically, starting with the end farthest from your anchoring element. The rope will rise into the sky as you work towards the anchored end, lessening the chances that any of your balloons will scrape against something and pop.

Finish the arch. If you have wrapped crepe paper all the way to the end of the rope, you should now be holding it in your hands. Cut or tear the crepe paper to end the wrapping, and tape it with a piece of transparent tape to keep it from unraveling at that end. Attach this end of the rope to your other anchoring element. Finally, if you are anchoring the arch with cinder blocks or similar items, use decorative items to obscure them. Your arch will sway back and forth with the wind, and stay upright thanks to the helium in the balloons, creating a festive, eye-catching motion effect.

Although floral elements are too heavy to run along this type of balloon arch, they are ideal for hiding the cinder blocks at either end.

Each anchoring element can be dragged to create a taller and thinner or lower and broader arch, so experiment until it looks right to you.